Humans use language as a basic means of communication. Nowadays, language is similarly used when humans communicate with devices. As such, machine recognition of natural language is an important topic, such as a speech recognition system of a vehicle.
It is very difficult to fully understand and respond to all natural languages that people use. For this reason, natural languages have been translated into meaningful languages only within a group (hereinafter referred to as “domain”) created by experts by organizing necessary meanings and corresponding expressions in a usage environment.
For example, in a case where a command that a user speaks is recognized and the recognized command is stored in the domain built in advance, a system may respond in an appropriate manner to the command of the user. However, if a keyword or a sentence is spoken but not defined in the domain, the command is not analyzed correctly. Because the system cannot find a service domain corresponding to the entered command, the system might perform a knowledge search (i.e., web-based search) to decipher the user's intent.
Problematically, if a web-based search is performed when the user's intent is not grasped, the system may provide contents that do not meet the user's intent or may interrupt a voice service due to error. In this case, the user determines that the system fails, and the user is not satisfied with the system. As such, the system may only provide the voice service to the user when the defined form and structure of the spoken sentence is completed. If the user does not know how to use or fails to speak the finished sentence, the voice service fails.